The invention relates generally to monitoring the delivery of fluid through a conduit, and more particularly, to monitoring the impedance to fluid flow in a fluid delivery system.
Fluid delivery systems for infusing fluid to a patient typically include a supply of the fluid to be administered, an infusion needle or cannula, an administration set connecting the fluid supply to the cannula, and a flow control device, such as a positive displacement infusion pump. The cannula is mounted at the distal end of the flexible tubing of the administration set for insertion into a patient's blood vessel or other body location to deliver the fluid infusate to the patient. One commonly used flow control device is a linear peristaltic type pump having several cams and cam-actuated fingers that sequentially occlude portions of the flexible tubing along a pumping zone to create a moving zone of occlusion. The peristaltic action forces the fluid through the tubing of the administration set to the cannula and into the patient.
During an infusion procedure, events may occur that interfere with the proper administration of the infusate to the patient, such as an occlusion of the administration line. It is desirable to detect these conditions as soon as possible so that they can be remedied. A commonly used clinical technique for detecting such conditions and for evaluating fluid delivery system status is to monitor the pressure in the fluid delivery conduit. An increasing pressure may be interpreted as an occlusion.
A difficulty in determining fluid delivery system status through monitoring the downstream pressure alone is the slow speed at which pressure builds when the system is operating at a low flow rate. At low flow rates, the energy per unit time introduced into the flow path is very small. This causes difficulty in detecting a fluid line fault based on the pressure response as it may take a considerable amount of time for the pressure to build up enough to exceed a threshold and indicate an occlusion. Lowering the threshold pressure level at which a fault is indicated will cause detection to occur earlier; however, it has been found that this approach can have the effect of increasing the false alarm rate. With a relatively low pressure threshold, patient movements such as coughing, sneezing, and sitting up can cause the pressure to exceed that threshold momentarily and may be falsely interpreted as a fluid delivery system fault.
Many developments have occurred in the analysis of the pressure existing in the fluid delivery conduit to detect fluid faults. For example, flow perturbations have been used to determine fluid delivery system status based on the pressure response to those perturbations. Other flow patterns have been applied for the purpose of generating a larger pressure response signal to determine fluid line status. However, problems of offset pressure and slow response times to low flow rates still exist in addition to the adverse effect some of these techniques have on flow uniformity.
As has been noted in U.S. Pat. No. 4,898,576 to Philip, the measure of the resistive part of the fluid line impedance can be used to monitor the condition of the fluid line. One technique used in actively monitoring the resistance, rather than merely waiting for pressure to build up, is the alteration of the flow rate. The change in the pressure over the change in the flow rate has been found to accurately indicate the resistive part of the fluid impedance in the system when adequate time is allowed for the pressure to reach equilibrium at each rate. This technique has been found to be effective at higher flow rates with their accompanying higher pressures. A change in these higher flow rates is accompanied by a rapid and measurable change in pressure. Because of the rapid pressure response to the flow rate changes, the flow rate can be varied about the selected flow rate without any significant clinical effect on flow uniformity.
However, at lower flow rates, the clinical requirement of flow rate uniformity restricts the magnitude of the perturbation that can be imposed on the fluid line. It is thus undesirable to alternate between different flow rates to obtain different pressure responses for determining resistance due to the detrimental effect on flow uniformity the flow changes would have as well as the relatively long length of time required to obtain those pressure responses.
Because pressure is used in determining resistance, unknown pressure offsets can have an undesirable effect of making accuracy in resistance estimates difficult to obtain. Additionally, other factors, such as those caused by other pumps impressing flow and resulting pressure responses on the fluid line, can result in inaccuracy in resistance measurement. Some increased immunity to such other factors is desirable as well as a means for compensating for offset pressure.
A further consideration in monitoring fluid line status is the update rate of the information presented. In the case where averaging or other techniques are used in processing pressure data, the update rate may be relatively slow at low flow rates. However, it would be desirable to reduce the delay so that more current data is available to detect faults in the fluid line.
Hence, those skilled in the art have recognized a need for a fluid delivery monitoring system that can detect a fluid delivery fault condition faster than prior systems at low flow rates and that can compensate for the existence of offset pressure while maintaining clinically acceptable flow patterns. Additionally, it has also been recognized that there is a need for a system that is less sensitive to other sources of pressure changes in the conduit such as those caused by other pumps on the same fluid line. It is further desirable to have a data update rate that can assist in detecting an adverse situation faster than the patient's physiological response to the drug being infused. The present invention fulfills these needs and others.